When trying to figure out how to test for a feature using the flask
config object as a parameter, I couldn't find a test for this feature
flag being used, so I went ahead and added one.

By defining a new config value that flask_pydantic looks for
(FLASK_PYDANTIC_VALIDATION_ERROR_RAISE), we can support raising an
exception when we have a validation error. If that is set and the
exception is raised, by default, Flask will respond with a 500 error.
However, by coupling that feature with `Flask.register_error_handler`,
the user can catch the thrown exception and return whatever custom error
response the user wishes. Fixes bauerji#45.